Dual-fuel combustion, where fuels of different reactivity are injected in the combustor separately in time or space, is important for some practical applications. This study employs Large Eddy Simulation (LES) coupled with the Doubly-Conditioned Moment Closure (DCMC) model to investigate dual-fuel flames, where liquid n-heptane ( C 7 H 1 6 ) is injected into a lean premixed methane/air ( C H 4 /air) swirling stream. Two premixed equivalence ratios, ϕ pmx = 0.14 and 0.56, are studied, representing conditions below and above the lower flammability limit (LFL) of methane. Cold flow simulations and comparison with experimental velocity data suggest that the flow field is reasonably well predicted. Combustion simulations then explore the impact of ϕ pmx on flame structure, with analysis performed in both physical space and conditional scalar space of DCMC. The results show good agreement with experimental observations in terms of flame shape and reaction zone location. The findings underscore the capability of LES-DCMC to capture key features of complex, multi-modal dual-fuel combustion systems.

One of the key ingredients in spray combustion is the turbulence effect on the droplet dynamics, especially the vaporisation process. In the present work we analyze the droplet interaction with the one-dimensional counterflow superimposed with external pulsations. The solution demonstrates that the pulsating component can significantly alter the spatial distribution of the evaporation intensity, making the mass source more uniformly distributed. The similar effect can also be expected under generally real turbulent conditions. Parametric studies are also conducted, including the oscillation amplitude, frequency, initial droplet diameter and the background flow strain rate.

A numerical study of the dynamics and stability of a two-dimensional flame front stabilised on a flat porous burner has been carried out within the framework of a diffusion-thermal model with one-step kinetics. Numerical results showed that the model successfully describes both small oscillations near the stability loss boundary and relaxational oscillations observed far from the bifurcation point at a low flow rate of a fresh mixture, which is also confirmed by comparison with the experiment. This indicates that a one-step diffusion-thermal model can be used to describe complex multidimensional nonlinear wave phenomena at the flame front. The influence of heat loss and burner width on the dynamics of the flame front and its structure is also studied. It is shown that the amplitude of flame front oscillations and its average position along the burner symmetry axis do not depend on the value of heat losses in the relaxation oscillation region, provided that the burner width is larger than the critical value. In addition, it is found that the frequency of oscillations in the central section of the burner in this case weakly depends on the heat losses in the whole area of observation of diffusion-thermal oscillations. Comparison of numerical and experimental data indicates that the diffusion-thermal model can be used to estimate the critical width of the burner, as well as the parameters of the flow of inert gas used to cut off the flow of reagents from the external environment. All this is necessary for the correct application of the method of verification of detailed reaction mechanisms based on a comparison of the characteristics of non-stationary pulsating combustion modes near the surface of a porous burner.

In this work, the current operation of a direct-flame and walking-beams type reheating furnace for conventional steel slab was optimized by means of mathematical simulation in a transitory state. The Navier-Stokes equations, the energy equation, the realizable k-ε turbulence model, the species transport equation and the P-1 thermal radiation model were numerically solved. The numerical simulation was validated with temperature measurements in plant inside the furnace, as well as with the rated power delivered by the different burners zones. The results indicate that the reduction of the air: methane ratios was sufficient to improve the thermal distribution of the furnace without affecting the exit temperature of the slab. Likewise, a more ordered fluid dynamics structure was obtained from the reaction products towards the furnace outlets. On the other hand, the predicted zones by simulation as feasible for greater iron oxide scale accumulation under the current operating plant conditions, were verified by visual inspection during the annual furnace maintenance shutdown. Finally, it was found that the generated atmosphere inside the furnace from the reaction products for the proposed conditions, is less likely to affect the iron oxidation reaction on the surface of the slab.

The ignition of spray flames is a process of stochastic nature, especially relevant to the high-altitude relight of aeroengines. The present work aims to quantify the role of spray inhomogeneities in a realistic droplet-laden ignition set-up through Direct Numerical Simulations (DNS). Computations of the ignition process of weakly turbulent ethanol spray flows are performed using complex chemistry, a realistic energy deposition model and a polydisperse description of the spray. A good qualitative agreement of the flame radius evolution is found between the DNS and experiments, and both long-time failure and ignition modes are recovered with comparable radii, hence reproducing the experimentally-observed range of behaviour of ignition kernels. In contrast to experiments where the presence of large droplets at the spark impact both the actual energy deposited in the flow and the local fuel-air equivalence ratio, the effect of both parameters on the ignition outcome are assessed independently in this work by carefully choosing ignition locations in the flow representative of time-wise fluctuations. The minimum ignition energy (MIE) representative of the global flow condition is then evaluated using results from the DNS simulations and flow and spark characteristics. In order to understand the origin of the ignition stochasticity observed experimentally, local values of the fuel-air equivalence ratio Φ t and turbulence level u ′ at the spark plug are measured in the simulations using an averaging in volumes of size Δ b . It is found that Φ t best correlates to the MIE at three different locations in the domain for a size around 4 mm, which corresponds approximately to the kernel size at 0.1 ms, that is, the typical time scale of plasma processes and right when the flame becomes toroidal. The results give support to the experimental conjecture that stochasticity is largely due to the spatial fluctuations of Φ t induced by polydispersity and in a lesser extent to those of u ′ .

The reaction zone structure of spontaneous reaction wave at steady subsonic and supersonic speeds is discussed using numerical simulation and theoretical approaches. The main objective of this study is to develop the mathematical model of the spontaneous reaction wave propagating at a steady speed and to clarify the relationship between the reaction zone structure and non-dimensional parameters. A one-dimensional direct numerical simulation (DNS) of the spontaneous reaction wave propagation is performed, and the modeling assumptions are discussed based on the numerical results. The non-dimensional parameters, the Damköhler number and a parameter related to compressibility defined by the Mach number of the spontaneous reaction wave velocity, are introduced into the modeling. The mathematical model is developed using asymptotic techniques, and the solution of the reaction zone structure is analytically derived using Newton’s method. The relationship between reaction zone structure of spontaneous reaction wave and non-dimensional parameters is discussed based on the analytical solutions.

This paper examines a new Conditional Source-term Estimation (CSE) implementation in which an integral inversion is performed for species mass fractions without any chemistry tabulation. To tackle the numerical stiffness due to chemistry, a constant-pressure chemical reactor is considered in conditional space. The objective of this work is to investigate this new method in CSE by comparing its numerical performance (accuracy of predictions and computational efficiency) with previous CSE implementations using tabulated chemistry. For the first time, the constraints for mass and mixture fraction conservation in conditional space are included in the inversion process. The investigation is performed in a Reynolds Averaged Navier-Stokes (RANS) framework using a well-documented turbulent flame with detailed measurements in conditional and physical spaces for many reacting scalars. A chemical mechanism including 16 species and 35 reactions is incorporated. CSE predictions of conditional and Favre averaged temperature and different species mass fractions are analysed. CSE with full chemistry performs as well as CSE with tabulated chemistry, if not slightly better. Numerical errors are discussed. The new CSE implementation is made possible by the proposed numerical treatment of stiffness due to chemistry and first-order Tikhonov regularisation technique with additional physical constraints. The new approach is found to be more CPU intensive than CSE with a pre-tabulation technique, but as a preliminary analysis, the computational cost of CSE with direct integration of a chemical mechanism appears to be much smaller than the run time associated with CMC for a similar set-up. Further investigation is required to refine this initial conclusion. These results open new opportunities towards CSE simulations that involve fuel blends and more complex operating conditions for example with spray and multi-stream inlets with non-uniform compositions for which accurate chemistry tabulations are difficult to generate.

Through energy and exergy analysis, the present work revisits the combustion processes generated using different fuels (CH4, C3H8, H2, LPG, Natural gas and Biogas) under the same operating conditions. First, numerical computations, in which the turbulent dynamic k-ϵ model coupled with a probability density function (PDF) approach together with different sub-models that ensure the reliability of the results, are used. Then, the First and Second Laws of thermodynamics are applied to all fuel combustion cases considered previously to predict the energy loss, exergy destruction and their corresponding efficiencies. Next, variations in the air factor, the air inlet temperature and the fuel inlet temperatures are considered to analyse the response of combustion system performance as well as energy and exergy losses for the different fuels under investigation. The results show an amelioration of combustion parameters for the mixture of fuel and air using values of λ greater than 1. Also, the preheating of the inlet air temperature has a positive effect on energetic parameters. However, preheating the inlet fuel does not take a considerable role in the efficiency amelioration and loss reduction of the combustion system. It further turns out that biogas features higher energy efficiency compared to other fuels, at approximately 89.64%, while hydrogen fuel exhibits higher exergy efficiency at around 78.70% followed by biogas with 74.96%.

Partially-premixed flames (PPFs) incorporate effects of both premixed and non-premixed types of reaction zones. The modelling of PPFs using manifold-based model reduction methods faces some inherent difficulties due to the underlying assumptions of a-priori identification of the type of combustion system. In this work, the reaction–diffusion manifold (REDIM) model reduction method is applied to study PPFs. The REDIM method requires minimal prior knowledge about the type of combustion system, which makes it a suitable method for studying PPFs. It allows incorporating system-specific diffusion (gradients) terms in a generic way so that the manifold can evolve according to the diffusion related information provided by the combustion system. In this way, a prior identification of the type of combustion system is no longer needed. This work utilises an iterative methodology to generate REDIM chemistry tables so that the reduced manifold can be iteratively converged very close to the detailed manifold according to the gradients of the reduced coordinates provided by the physical combustion system in each iteration step. In addition, a new method is proposed to provide the gradient estimates of the reduced coordinates during the generation of REDIM from the scattered gradient data in REDIM reduced CFD calculations. Laminar triple flames, a special case of PPFs, with two types of mixture fraction gradients are selected as the target cases to assess the presented iterative methodology. REDIM reduced calculations are compared with simulations based on detailed finite-rate kinetics. It is found that in the final iteration steps, temperature and all considered major and minor species mass fraction profiles are very well predicted by the REDIM reduced calculations.